1. Field of the Invention
The present invention relates to a four cycle, 12-cylinder engine having an intake system adapted for "intake resonant supercharging".
2. Description of the Prior Art
In the intake system of an internal combustion engine, it has been recognized that pulsation of the intake air is produced in the intake passage in response to the operation of the intake valve. The pulsation is considered to have a natural frequency which is determined by various factors such as the length of the intake passage between the intake valve and the open end to the atmosphere, and the sectional configuration of the intake passage.
Various efforts have been made in the past to utilize this phenomenon in obtaining a supercharging effect. This can be done by designing the intake passage such that the natural frequency of the pulsation is matched with the operating frequency of the intake valve so that positive pressure prevails around the intake port in the intake stroke under an engine speed in which a high engine output is desired. The positive pressure around the intake port is effective to increase the intake charge to thereby provide an increased volumetric efficiency. The supercharging of this type may be called "intake resonant supercharging" because it utilizes intake air resonance in the intake passage.
In applying the principle of intake resonant superchaging to a multiple cylinder engine, it has been proposed to divide the cylinders of the engine into a plurality of groups. In grouping the cylinders, the sequence of the intake strokes is taken into account such that in terms of the sequence of the intake strokes the cylinders in one group are not adjacent to each other. In other words, grouping is made such that between the intake strokes of two cylinders in one group there occurs an intake stroke in another cylinder group. The intake ports of the cylinders in one group are connected through branch intake passages with a common intake passage which may constitute a surge tank. The common intake passage for one cylinder group is usually separated from the common intake passage for other cylinder groups so that the branch intake passages and the common intake passage of each cylinder group provide a length of the intake passage for the intake air resonance. A shut-off valve may be provided between the common intake passages of the plurality of cylinder groups so that the common intake passages may be communicated with each other when desired to provide another length of the intake passage for the intake air resonance. With this arrangement, the intake resonant supercharging effect can be obtained under a plurality of engine speeds. An example of this arrangement is disclosed by the U.S. Pat. No. 4,829,941 issued to M. Hitomi et. al.
Recent trends in automobile engine design are to provide an increased number of cylinders so that engine output torque fluctuation can be minimized. It is predicted that new 12-cylinder engines will be developed for high class passenger cars. In designing such 12 cylinder engines, it is also desirable to apply the aforementioned intake resonant supercharging principle. However, such supercharging system generally has complicated and bulky intake passages so that it is not easy to apply the intake supercharging to a 12-cylinder engine.